Electrical devices that convert an AC input into a steady DC output are generally known as static power converters (SPCs). SPCs typically include a rectifier, a filter and a DC bus. The rectifier rectifies AC power it receives from a utility line or other AC power source into DC power using diodes or power semiconductors. The filter generally includes one or more DC smoothing capacitors connected across the DC bus, which carries output from the rectifier, for smoothing the resultant DC output. The DC filtering capacitors smooth pulsating DC output from the rectifier by absorbing peak currents and ripple currents while providing a source of constant DC voltage. Many conventional filters include one or more capacitors electrically connected in series and/or in parallel. When one of these capacitors shorts, other capacitors serially connected to the shorted capacitor are often overstressed with an in-rush of current, which causes their failure. Such failure is often explosive, thereby causing fire or shock damage to the system.
SPCs have a variety of uses, such as providing input power to an inverter as part of an inverter drive system. Inverters are known in the art as devices that generally receive a DC power source at their input and provide either a single phase or a polyphase AC output. Variable voltage inverters and current source inverters are examples of inverter types known in the art for providing a controlled AC output. In a conventional inverter drive system, an SPC receives raw AC power and converts it into a steady supply of DC power, which the inverter uses to provide controlled AC output to a load. Such systems are common and may be used, for example, to drive an AC motor or to provide power for AC uninterruptible power supplies (UPS's).
One widespread use of an inverter drive system including an SPC is to provide adjustable output power as an adjustable speed drive. Such an adjustable drive system may control, for example, a DC brushless motor, an AC induction motor by acting as a vector controller, or an AC induction motor by acting as an AC variable frequency controller. For instance, a variable frequency drive (VFD) type of adjustable drive system may control an AC synchronous motor by varying the AC output frequency using pulse width modulation (PWM) techniques. Such VFD devices are popular due to their efficiency, energy savings and reliability. Another widespread use of inverter drive systems is for UPS devices, which are increasingly popular for providing a stable AC power supply to sensitive electronic devices, such as computers and printers.
An example of an inverter drive system is shown in FIG. 1, which shows a voltage fed inverter system 10 as is known in the art for providing controlled AC power output. The system 10 includes a rectifier bridge 12, a DC bus 14, a filter 16, and an inverter circuit 18. The rectifier bridge (REC) 12 receives three-phase power from a power source (not shown) via input terminals 20 and converts it to DC power via diodes 22. Typically, between each input terminal 20 and REC 12 there is an in-line fuse 24, which opens in the event of over-current into REC 12. Filter 16 generally includes one or more smoothing capacitors 26, 28 that are often electrically connected in series. Capacitors 26, 28 smooth the potential across DC bus 14 to provide a relatively constant output voltage. The voltage across DC bus 14 provides a controlled input to inverter circuit 18. Inverter circuit 18 is controlled, such as via PWM techniques, to provide a controlled AC power output.
In such conventional systems, DC bus fuse 30 is provided along one or both sides of DC bus 14. If the current through DC bus 14 exceeds a pre-determined level, DC bus fuse 30 opens and disables inverter system 10. For example, if a transistor (not shown) of inverter 18 shorts such that the positive and negative lines of DC bus 14 are directly connected in certain switched modes, an over-current will result through DC bus 14 causing fuse 30 to open. In another example, an over-current condition may occur as a result of one of capacitors 26, 28 shorting; however, this may not occur prior to the matched capacitor also failing. For example, if capacitor 26 shorts, its serially connected matching capacitor 28 receives an in-rush of current that may exceed its rating. Because the increased current to capacitor 28 may not exceed the rating of fuse 30, fuse 30 will remain intact and closed. As such, capacitor 28 may quickly fail before fuse 30 opens. Because of the increased stress to capacitor 28, its failure is likely more severe than the first failed capacitor 26. Thus, the failure of the second capacitor 26 may be hazardous, which could cause fire or shock to the system and create unsafe conditions.
Accordingly, a need exists for a DC bus circuit that prevents hazards associated with failure of a smoothing capacitor. Further, a need exists for a DC bus circuit that prevents the failure of a matched capacitor in the event one capacitor of the series shorts.